1. Field of the Invention
This invention relates to a power source for supplying high voltage to a load and, more particularly, to a high voltage resonant DC/DC converter adapted as a power source for an X-ray tube, etc.
2. Description of the Prior Art
An X-ray tube is generally operated by high voltage ordinarily ranging between 100 and 400 KV, with a larger power consumption than 100 KW. With a conventional high voltage converter for supplying high voltage to a load such as an X-ray tube, the common technique used to derive the high voltage has been to step up a commercial power source by means of a transformer whose core is constructed of laminated silicon steel plates. Procurement of high voltage and high power from a commercial power source operated at a low frequency requires an extremely large transforemer, making it necessary to provide a large high voltage converter.
To cope with the above-mentioned difficulties, studies have been conducted in recent years to miniaturize the transformer and reduce the capacity of the power source for the high voltage converter by applying a converter having as high a switching frequency as several hundred Hz for power conversion. In the above-mentioned power conversion, a large power of, for example, 1,000 amperes is generally switched. Therefore, a high speed gate turn-off (GTO) thyristor, used as a switching element in the converter, can have its operation frequency only raised to a level of about 1 KHz, if its switching loss is taken into account. Consequently, a limitation is imposed on the miniaturization of the transformer, thereby unavoidably resulting in the enlargement of the high voltage generator. In the above-mentioned large power conversion, difficulties arise in elevating the operation frequency of the switching element in view of the switching loss which may take place. Consequently, the miniaturization of the transformer has been considered unrealizable, if the power transmission efficiency of the transformer is taken into account.
Since the core undergoes magnetic saturation, a limit is imposed on the transformer magnetic flux density B (magnetic saturation is reached at about 8,000 gauss in the case of a silicon steel plate and at about 3,000 gauss in the case of a ferrite plate). For the transmission of high power, the cross-sectional area of the core has to be enlarged. The above-mentioned magnetic saturation has also been regarded as one of the causes which prevents miniaturization of the transformer.
As seen from the foregoing description, the application of a large transformer results in the enlargement of the whole high voltage converter, thus requiring, a great deal of work and time in the transportation and installation of a large high voltage converter. Moreover, such a large high voltage converter imposes further limitations on the selection of a site for its installation.